pinhole calculator

Why a Pinhole Calculator Matters

Pinhole photography is simple in concept and surprisingly unforgiving in practice. If the hole is too small, diffraction softens the image. If it is too large, geometric blur takes over. The sweet spot depends on the camera’s focal length, which is why a dedicated pinhole calculator is so useful.

With one quick calculation, you can get a practical starting point for three key decisions:

• How large to make the pinhole.
• What effective f-number your camera will have.
• How long your exposure should be compared to your meter reading.

How the Math Works

1) Optimal pinhole diameter

The calculator uses a classic optimization formula for a circular pinhole:

d = 1.9 × √(f × λ)

For visible light, a wavelength around 550 nm (green light) is a common default and works well for general photography. As focal length increases, optimal pinhole diameter increases too.

2) Effective aperture (f-number)

Once diameter is known, effective aperture is straightforward:

N = f / d

Most pinhole cameras land somewhere between f/100 and f/300. That means long exposures are normal, especially indoors or at lower ISO values.

3) Exposure scaling

Exposure changes with the square of the f-number ratio. If your meter gives a shutter speed at f/16, the pinhole exposure is:

t_pinhole = t_metered × (N_pinhole / N_ref)²

This gives a strong baseline. For film and paper, additional reciprocity failure correction may be needed for longer exposures.

Using This Calculator in Real Builds

Step-by-step workflow

• Measure the distance from pinhole to film/sensor plane in millimeters.
• Calculate and make the nearest clean pinhole size possible.
• Meter the scene at a convenient reference aperture (such as f/16).
• Use the calculated pinhole f-number to convert to final shutter time.
• Add reciprocity correction when working with film or paper for multi-second exposures.

Practical build tips

• Use thin metal (shim stock or beverage can aluminum) for cleaner holes.
• Polish gently and remove burrs; edge quality affects sharpness.
• Keep the pinhole perfectly flat and centered to avoid uneven focus falloff.
• Test brackets around your computed value (for example ±10%) to find your preferred look.

Example

Suppose your camera depth is 50 mm. At 550 nm, the calculator returns an optimal pinhole near 0.31 mm, giving an aperture around f/160. If your meter says 1/125 second at f/16, your pinhole exposure is around 0.8 seconds before reciprocity correction.

That quick conversion is exactly what makes a pinhole calculator handy in the field.

Common Mistakes to Avoid

• Mixing units (nm, mm, inches) without conversion.
• Assuming metered times already include reciprocity correction.
• Ignoring pinhole quality: shape and burrs matter as much as diameter.
• Forgetting that very wide pinhole cameras can vignette due to geometry and material thickness.

Final Thoughts

Pinhole photography rewards patience and experimentation. This calculator gives you a reliable technical baseline, but artistic preference still matters. Some photographers intentionally deviate from “optimal” diameter for softer rendering or longer exposure character.

Use these numbers as your launch point, then test, refine, and build your own pinhole style.